Inkjet printers, and certain other types of incremental printers, are inherently capable of a very limited number of tonal levels. To achieve a moderate range of tonal variations in such printers, halftoning or dither masks are used to transform or “render” input variations in intensity in the form of spatially varying densities.
The eye integrates the spatial variations to, in effect, reconstruct a semblance of continuous-tone capability. The halftoning process, in incremental printers, sometimes employs masks known as dither masks or halftone masks, and the preparation of these masks is a matter of importance in terms of preparation time, data storage, and final output image quality.
To achieve vivid colors in inkjet printing with aqueous inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the water base—by evaporation (and, for some printing media, absorption)—and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise. These include so-called “coalescence” and “bleed” of one color into another (particularly noticeable at color boundaries that should be sharp), “blocking” or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and “cockle” or puckering of the printing medium. Various techniques, discussed below, are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects.
(a) Prior heat-application techniques—Among these techniques is heating the inked medium to accelerate evaporation of the water base or carrier. Heating, however, has limitations of its own; and in turn creates other difficulties due to heat-induced deformation of the printing medium.
Glossy stock warps severely in response to heat, and transparencies too can tolerate somewhat less heating than ordinary paper. Accordingly, heating has provided only limited improvement of drying characteristics for these plastic media.
As to paper, the application of heat and ink causes dimensional changes that affect the quality of the image or graphic. Specifically, it has been found preferable to precondition the paper by application of heat before contact of the ink; if preheating is not provided, so-called “end-of-page handoff” quality defects occur—this defect takes the form of a straight image-discontinuity band formed across the bottom of each page when the page bottom is released.
Preheating, however, causes loss of moisture content and resultant shrinking of the paper fibers. To maintain the paper dimensions under these circumstances the paper may be held in tension, but this too induces still other types of image defects requiring yet further innovation to overcome them.
(b) Printmode techniques—Another useful technique is laying down in each pass of the pen only a fraction of the total ink required in each section of the image—so that any areas left white in each pass are filled in by one or more later passes. This tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a “print mode”. Heretofore most efforts in design of print modes have focused upon difficulties introduced by regularity or repetition of patterns previously regarded as inherent in printmode techniques.
For example, some print modes such as square or rectangular checkerboard-like patterns tend to create objectionable moire effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise in dithering subsystems sometimes used to help control the paper advance or the pen speed.
More recently, however, attention has turned to use of random or more-properly “randomized” patterns. These are introduced in the coowned patent documents listed earlier and will be discussed in greater detail shortly.
(c) Known technology of printmodes: general introduction—The pattern used in printing each nozzle section is known as the “printmode mask” or “printmask”. The term “printmode” is more general, usually encompassing a description of a mask, the number of passes required to reach full density and the number of drops per pixel defining “full density”.
One particularly simple way to divide up a desired amount of ink into more than one pen pass is the checker-board pattern mentioned above: every other pixel location is printed on one pass, and then the blanks are filled in on the next pass. This pioneering strategy was quickly recognized as inadequate for highly demanding competitive modern printers because of ink coalescence along diagonals, inability to correct moire phenomena, and also the appearance of so-called “banding” or evident boundaries between abutting ink swaths.
To reduce such horizontal banding problems (and sometimes minimize the moire patterns) discussed above, a print mode may be constructed so that the paper advances between each initial-swath scan of the pen and the corresponding fill-swath scan or scans. In fact this can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
This technique tends to distribute rather than accumulate print-mechanism error that is impossible or expensive to reduce. The result is to minimize the conspicuousness of—or, in simpler terms, to hide—the error at minimal cost.
All of these strategies are now well known and have been elaborated to a very great extent, including the use of space- and sweep-rotated printmode masks, autorotating printmode masks (in which rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes), and steeply angled, separated mask-pattern lines. An extensive discussion of such methods appears in U.S. Pat. No. 5,677,716 of Cleveland. Analogous problems and solutions are known in regard to dither masks as well.
(d) Nozzle-variation effects—Different groups or areas of nozzles in a pen have different effects on image quality. Only a few techniques have been introduced for adapting printer control mechanisms to accommodate or even exploit such variations.
European Patent 730,967 of Nicoloff et al. explains that overlap between two different-color inks, during a single scan, can be prevented by operating only a section of each nozzle array (i. e., pen) during such a single scan. It appears that the selection of the section to be operated, in Nicoloff, is consistent.
United Kingdom Patent 2,302,065 of Nobel et al. deals exclusively with text printing. It teaches variation of printhead alignment with respect to the lines of text, from page to page or at other intervals—so that top and bottom nozzles can be used about equally and also causes wear of middle nozzles to be more uniform.
As pens are fabricated by automatic techniques that continually increase in efficiency and speed, new kinds of nozzle variations continue to appear. Further development in the area of dealing with such variations is therefore needed.
(e) A new generation of banding problems—Heretofore, however, all these many ingenious stratagems have fallen short of eliminating banding. This is particularly true in the midtone range where small repetitive patterns, even subtle patterns, often become very objectionably conspicuous. Such difficulties are known in regard to both dither masks and printmasks.
Particularly as to the latter, the current generation of efforts includes the work outlined in at least the Serra and Chang documents to eliminate such problems by “randomizing” of masks, i. e. building masks by pseudorandom processes. Generally speaking, both these groups of workers teach the possibility of operating a randomized mask-building process in real time, in a printer—but reject that possibility, teaching away from it in favor of human inspection of computer-generated candidate masks, for best esthetics of printed results.
Chang suggests a balance between ink-media artifacts (which he associates with such esthetic choice by a live person) and mechanical artifacts (which he associates with randomization as a curative). Chang accordingly teaches that the inspection step is essential at least for the present.
The efforts of both Serra and Chang are distinct and important improvements over the prior art. With respect to their largest objectives, however, these efforts have foundered on the persistence of conspicuous and distinctive—even though substantially random—patterns that repeat dozens of times across a typical image.
In hindsight, the reason behind such persistence is clear. Operation of printers using these newest, randomized masks amounts to a superposition of a repeating mask pattern with a constant pattern of pen nozzle misdirections, weak or failed nozzles, and other flaws of static character. The system convolves the same mask patterns with the same nozzles over and over.
The resulting granularity is generally far larger than individual pixels or inkdrops. Although some shapes can be reduced for a particular image by tinkering with mask patterns, this approach too is typically prohibitive with common images—which are ordinarily very complex.
Of course it is not intended here to unduly criticize the contributions of these skillful and talented workers in this field, who have produced important and useful advances. Yet that effort leaves room for improvement.
Indeed, from the perspective of a skilled person in this field, intuitively it might appear that only a mask on the order of a major fraction of a full-page image would suffice to truly eliminate conspicuous patterning. From that perspective the results of these copending patent documents appear to be perhaps some two orders of magnitude away, in terms of mask size, from such an expectably successful size standard.
Earlier masks have been small. In the printing-medium advance direction (vertical for a portrait-format image), mask height customarily has been equal to the number of nozzles divided by the number of passes, or an even smaller quotient. In the scan-axis direction (horizontal), mask width ordinarily has been equal to the height or less—typically eight to thirty-two pixels.
These numbers represent very small fractions of a full-page image width at the modern resolution standard of twenty-four pixels per millimeter (six hundred pixels per inch). Even increasing mask size to, say a third or a half of full-page width calls for an increase by a factor of some fifty to seventy.
That is a daunting prospect, since such very large masks require very large memory—and different masks are required for various conditions, particularly including type of printing medium as well as printing speed. The crosscombinations of these conditions rise quickly to require at least some eight or twelve different masks. The cost of storing not one but numerous manufacturer-supplied multimegabyte-size masks, and the competitive handicap of requiring such mask storage in a computer attached to a printer, are all but prohibitive.
The picture as seen from the current generation of advances is similarly adverse with regard to generation of such large masks in the field. While the Serra results may provide somewhat better image quality, both the Serra and Chang algorithms appear to be too expensive computationally to implement in onboard processing systems of a printer, or even in a printer driver operable in most personal computers.
With such a strategy, a mask-building program might be started on a workstation to operate overnight, and the next day one or two more-or-less valid solutions might be found to test with real conditions. This alone points up a major drawback at a conceptual level: it is analogous to assigning a lower primate to press typewriter keys essentially at random—with a person necessarily inspecting the results from time to time to see whether what was typed makes sense.
If not, the person discards all of that and the animal starts again—a painfully inefficient procedure. Moreover, notwithstanding commitment to such a computationally intensive method, as already indicated the masks built in this way were rather small, on the order of sixteen by thirty pixels.
Another of the current generation of advances in the printmasking art, the Doron document mentioned earlier, proposes building a good mask automatically at each effort, and suggests that this may be accomplished within a printer in the field and in real time (i. e., when or shortly before the mask is needed). Doron indicates that a differently configured mask is selected for each printing operation—that is, for each printer pass in a repeating series of, say, four or five passes—and his approach evidently thus entails use of not one but several discrete masks for each image.
The printer selects a different one of these masks for each pass, which is to say that different masks are applied to printing of different image portions. In the earlier procedure of building a mask, Doron's procedure operates in a three-dimensional space that is a vertical stack of pixel grids, each successive grid or plane representing a higher layer of inkdrops applied in succeeding passes, or by later-arriving nozzles in a single pass.
Doron appears to introduce randomness at two stages of his procedures. First, in the mask-building stage he randomly selects a series of vertical columns in the stack—e. g., columns that underlie particular pixel positions in the topmost grid—for processing, one column at a time.
In processing each column he fills-in printing parameters for the several grids in the stack, observing selection rules that refer to columns which have been previously filled in. The selection rules appear to result in an essentially deterministic series of selected “parameter” numbers down the column, given the columns filled in previously. The random order of column selections in this way controls the final overall three-dimensional array of parameters.
Second, at run time Doron's control program randomly selects one of a number of the randomly generated masks for use in each “printing operation” as mentioned above. The degree of randomness in his pseudorandom system appears to be fixed by these two processes.
Although it uses plural masks in conjunction for printing of each image, his system appears to require relatively modest quantities of data storage. It requires more than just the storage for one mask, but less than would be required for permanent storage of all necessary masks, and a small amount in total because each of his masks is rather small.
The efficacy of the system is unclear, in that it attempts to rely upon continuously shifting among several available masks to break up patterning but the individual masks are small. Doron's mask height (i. e. a y dimension in the pixel grid) is his pen height, preferably one hundred twenty-eight pixels; however, his most-highly preferred mask width is said to be only thirty-two pixels.
Thus if the number of masks is also rather small, some potential for repetitive patterns breaking through may remain. On the other hand if the number of masks is adequately large and the selection truly randomized, then the system may exhibit adverse effects of highly random masking. Since such effects are part of the recognitions driving the present invention, they will be explained further in the following sections.
(f) Conclusion—As shown above and elaborated in the following sections, various obstacles of computing time, data storage, a fixed degree of randomness and repetitive small or narrow (although randomized) mask patterns have continued to impede achievement of uniformly excellent inkjet printing—at high throughput—on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.